You start with 4800 grams uranium - 238. It's radioactive and goes through alpha decay. How many grams remain after 7 half-lives.

37.5g

Explanation:

The half-life is the time taken for a radioactive substance to decay to half its original composition;

 Original composition here is  = 4800g  of U-238

How many grams remain after 7 half-lives;

 After each life, the uranium reduces by half;

 Amount remaining  = 4800 x  

                                  = 37.5g

After 7 half lives, 37.5g of the original sample will remain.

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c-14 is carbon-14, or radiocarbon.   it is a radioactive isotope of carbon with an atomic nucleus containing 6 protons and 8 neutrons.